Potential gradient detector for electrophoresis

ABSTRACT

An on-column detector for electrophoresis samples based on the principles of potential gradient detection, in which the electrodes for detection are physically isolated from the electrophoretic separation process, but maintains the same electrical potential as the corresponding interior of the electrophoretic separation channel. Potential gradient detection is used to measure the applied electrical field at two points within the electrophoretic channel during electrophoresis. When sample components with conductivity different from the electrophoretic medium passes between these two points, it causes a change in the potential gradient between the two points, which would be sensed by the sensing electrodes of the detector and registered by a data acquisition system. The apparatus can make use of conventional separation channel as well as separation channels on microchips. In accordance with the present invention, a sensor with electrically conductive medium is added and connected to the separation channel via a conductive element on the surface of the separation channel.

FIELD OF THE INVENTION

The present invention is related to sample detection in electrophoresis.In particular, the present invention is related to conductivitydetection in electrophoresis.

BACKGROUND OF THE INVENTION

Capillary electrophoresis (CE) is a powerful analytical separationtechnique for the analysis of complex mixtures. In CE, an unknown sampleis introduced at an Inlet of a capillary channel filled with a buffersolution, and a high voltage is applied across a section of thecapillary. Different constituents of the sample migrate through thecapillary at different rates depending on their electrophoreticmobility's, and are separated into different zones. By detecting thechemicals passing through a part of the capillary or its outlet as afunction of time, and knowing the of the possible constituents, thechemical composition of the sample can be determined. A number ofdetectors have been developed for CE, including optical andelectrochemical methods. Electrochemical detection can be classifiedinto three main categories: amperometry, voltammetry and conductivitymeasurements. Conductivity detection is a non-selective detection modeand universally applicable. Analytes are detected because of theirdifferent conductivities to that of the background electrolyte.

One method to measure conductivity during electrophoresis is potentialgradient detection, which is accomplished by putting two electrodes inthe applied electric field for electrophoresis and detecting samplecomponents by measuring potential changes between these two electrodeswhen sample components are passing by. This method has been used forisotachophoresis (U.S. Pat. No. 3,941,678, 20 Feb. 1975 and U.S. Pat.No. 3,932,264, 13 Jan. 1976) and it has been mentioned that such amethod can be used in modem CE (F. Foret, L. Krivankova and P. Bocek,Capillary Zone Electrophoresis, chapter 7, p147-150). There are,however, problems for using this method in electrophoresis. Firstly, thesensing electrodes need to be inserted into the separation column orcapillary. The procedures are troublesome and tedious, especially if theinner diameter of the capillary is small, for example in the case ofcapillary electrophoresis (usually between 10-100 μm). The more seriousproblem is that the sensing electrodes are polarized duringelectrophoresis. In order to prevent formation of bubbles and depositson the electrodes so that the electrophoresis processes can be performedunder stable conditions and high sensitivity can be obtained, specialmeans have to be used, such as adopting v/F and F/v converters in theinstrumental design, reducing the areas of electrodes contacting withthe buffer solutions and adding nonionic surfactant. However, all thesemeans can only serve to alleviate, but can't eliminate completely theproblems encountered.

Therefore, conductivity detection is usually accomplished by measuringthe potential difference (signal) between two electrodes while passingthrough a small constant current (excited source). Several designs areused for conductivity detection in CE, i.e., on-column, end-column andcontactless structures. On-column detection cells (Anal. Chem., 1987,59, 2747-2749, U.S. Pat. No. 5,223,114, 29 Jun. 1993 and U.S. Pat. No.5,580,435, 1994) are usually made by inserting two sensing platinumwires into the separation capillary so that the sensing electrodes candirectly contact the electrolyte solution in the capillary. Althoughon-column conductometric detection works well, the question arises as tohow to produce such structures reliably and inexpensively. The morecommon practices are the use of end-column detectors (such as thosedisclosed in Anal. Chem. 1991, 63, 189-192, J. of CapillaryElectrophoresis, 1996, 1:1-11, U.S. Pat. Nos. 5,298,139, and 5,126,023),which have the advantage that the sensing electrode can be constructeddirectly at the outlet of the separation capillary. For end-columndetection, the correct alignment of the sensing electrode with theoutlet of the separation capillary is critical for success. However, thealignment is usually difficult due to the small inner diameter (10μm-100 μm) of the capillary.

Another solution offered in the prior art is contactless conductivitydetection (Anal. Chem., 1998, 70, 563-567). In this method, twoelectrodes are laid on the outside wall of the separation capillary.Therefore, no electrode is in contact with electrolyte solution.However, it is generally accepted that the contactless detection is notsensitive enough. Although their structures are varied, all the priordesigns should use their own excited source and considered the highvoltage applied for electrophoresis as a noise source.

Although the three techniques described above (i.e. potential gradientdetection, potential difference detection, and contactless conductivitydetection) are all based on the difference in conductivity between theelectrophoretic medium and the samples, potential difference detectionis the most widely used technique in capillary electrophoresis.Therefore, commercially available and commonly described conductivitydetection systems typically employ the potential difference detectionmethod.

OBJECT OF THE INVENTION

It is therefore an object of the present invention to provide aconductivity detection system which obviates the need to insertelectrodes into the separation channel.

It is another object to provide a conductivity detection system which iseffective and sensitive.

SUMMARY OF THE INVENTION

The present invention provides an on-column electrochemical detectorbased on the principle of potential gradient detection forelectrophoresis of samples in which the electrodes for detection arephysically isolated from the electrophoretic separation channel, butmaintain the same electrical potential as the corresponding interior ofthe electrophoretic separation channel. Since the sensing electrodes arenot in direct contact with the electrophoretic medium within theelectrophoretic channel, problems due to bubble and deposit formationare eliminated.

The apparatus can make use of conventional separation channel with aninlet end connected to an inlet reservoir and an outlet end connected toan outlet reservoir. In accordance with the present invention, a sensorreservoir with electrically conductive medium is added and connected tothe separation channel via a conductive element on the surface of theseparation channel. A sensing electrode is submerged in the electricallyconductive medium within the sensing reservoir. The conductive elementallows electrical potential from the interior of the separation channelto be transferred to the sensor reservoir without detectable bulk flowof electrophoretic medium or samples. Detectable bulk flow refers to themovement of solute or sample to an extent that there is detectableinterference or disruption to migration of the sample. This detectionmay be based standard detection methods or the method described in thepresent invention. In one embodiment, the conductive element is afracture in a separation channel made of fused silica tubing. In anotherembodiment, the conductive element is a thin layer of porous glass onthe wall of a capillary channel electrophoretic chip.

During electrophoresis, the channel and reservoirs are filled withelectrophoretic medium, and the ground and power electrodes from a powersupply are connected to the outlet and inlet reservoirs respectively.Sample detection is achieved by sensing the potential gradient betweenthe conductive element and the outlet end where the sensing andreference electrodes are respectively connected in the preferredembodiment. The distance between the element and the outlet end is alsopreferably as small as the length of the sample plug in order tomaximize sensitivity of detection and resolution.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and B are typical detection methods described in the prior art.

FIG. 2A is a schematic diagram of a capillary electrophoresis system asdescribed in one embodiment of the present invention.

FIG. 2B is an enlarged view of area G as shown in FIG. 2A.

FIG. 3A shows the electric field as detected by the data acquisitionsystem using the capillary electrophoretic system shown in FIG. 2Awithout sample.

FIG. 3B shows the electric field as detected by the data acquisitionsystem using the capillary electrophoretic system shown in FIG. 2A whensample is injected and separated into zones.

FIG. 4 is a schematic diagram of the microchip CE system to illustrateanother embodiment of the invention.

FIG. 5 is an enlarged view of area H as shown in FIG. 4.

FIG. 6 is a block diagram of a circuit for the detector to illustrateyet another embodiment of the present invention.

FIG. 7 is an electropherogram obtained using the system shown in FIG.2A.

FIG. 8 is an electropherogram obtained on microchip CE using the systemshown in FIG. 4.

FIG. 9 is a schematic diagram of yet another embodiment of the presentinvention.

DESCRIPTION OF THE INVENTION

The following detailed description describes the preferred embodimentfor implementing the underlying principles of the present invention. Oneskilled in the art should understand, however, that the followingdescription is meant to be illustrative of the present invention, andshould not be construed as limiting the principles discussed herein. Inaddition, certain terms are used throughout the following descriptionand claims to refer to particular system components. As one skilled inthe art will appreciate, companies may refer to a component by differentnames. This document does not intend to distinguish between componentsthat differ in name but not in function. For example, the pair ofelectrodes for electrophoresis are referred to herein as “ground” and“power” electrodes for clarity of description. It is understood by oneskilled in the art that the ground electrode may be at zero volts orfloating, and that the power electrode may be of positive or negativepolarity. For the same reason of clarity of description, the pair ofelectrodes for potential gradient detection are referred to herein as“sensing” and “reference” electrodes. It should be understood that the“sensing” electrode could be the same type as the “reference” electrode.Their positions can be exchanged with each other without affectingdetection results. When performing electrophoresis on microchip, atleast four electrodes are often needed for sample introduction andseparation. For ease of understanding, these electrodes are classifiedas “power”, “ground”, “sample” and “waste” electrodes. It should beunderstood that the exact potential on these electrodes are not fixed,and may be set up according to the needs of the user. The reservoirs onthe microchip have also been given the names “inlet”, “outlet”, “sample”and “waste” reservoirs for clarity of description. It should also beunderstood that the reservoirs can be used to contain different mediumdepending on the experimental conditions required. Furthermore, noparticular inlet and outlet reservoir structures are required ifmicroinstruments are used to load small quantities of samples directlyinto the inlet end.

In the following discussion, and in the claims the terms “including”,“having” and “comprising” are used in an open-ended fashion, and thusshould be interpreted to mean “including but not limited to . . . ”.Also, the term” or “couples” is intended to mean either an indirect ordirect electrical connection. Thus if a first device “couples” to asecond devices, that connection may be a direct electrical connection orthrough an indirect electrical connection via other devices, electricalconductive medium or connections. Capillary electrophoresis is used forpurposes of illustration. It should be understood by one skilled in theart that the same principles may be applied to other types ofelectrophoretic separations, using the teaching provided herewith.

FIGS. 1A and B show a section of an electrophoretic capillary tube,illustrating the principles of potential gradient and potentialdifference detection systems respectively as known in the art. In thepotential gradient detection system as shown in FIG. 1A, the two sensingelectrodes 10A and 10B come into contact with the electrophoretic mediumat two non-parallel positions along the longitudinal axis of theelectrophoretic channel 12, which is connected to a power sourcegenerating an electrical potential between ends 12 a and 12 b. Theportions of the electrodes in contact with the solution inside thecapillary tube is shown in dotted lines. In the potential differencedetection method, the two sensing electrodes 14A and 14B have to be incontact with the electrophoretic medium at exactly the samecross-sectional plane of channel 15 having an electrophoretic potentialbetween ends 15 a and 15 b. The resistance of the electrophretic mediummay be monitored using a small a.c. current between the sensingelectrodes. Because the sensing electrodes are within high electricfield during electrophoresis, bubble and deposit form on the surface ofthe sensing electrodes due to electrochemical reactions, which wouldaffect the electrophoretic process, and decrease the detectionsensitivity since the sensing electrodes are directly within theelectrophoretic channel.

FIGS. 2A and 2B show one setup of capillary electrophoresis (CE) basedon potential gradient detection constructed in accordance with thepresent invention. A 50 μm inner diameter fused silica capillary is usedas the separation capillary 32. A fracture 58 is made before the outletof the separation capillary 32. The distance L between the fracture 58to the outlet, usually between 0.1 mm to 5 mm, should be near or smallerthan the length of the sample plug injected into the separationcapillary 32 in order to obtain maximum resolution between separatedpeaks. The capillary 32 is then inserted into the buffer reservoirs sothat the outlet 37 of the capillary is connected to outlet reservoir 38,fracture 58 is submerged in sensor reservoir 34, and inlet 31 of thecapillary is inserted into inlet reservoir 30. Good insulation betweenreservoirs 34 and 38 is made by using an insulation layer 54. Runningbuffer solutions for electrophoresis are filled into the three bufferreservoirs as well as the bore of the separation capillary 32. Theground and power electrodes 46 and 26 are connected with the highvoltage power supply 20 to apply high voltages needed forelectrophoresis. Sensing electrode 44 is put in electrically conductingsolution 36 contained in sensor reservoir 34, and a reference electrode42 is put in the solution 40 contained in outlet reservoir 38. Betweenelectrodes 44 and 42, two resistors 48 and 50 are used to sample thepotentials between electrodes 44 and 42 to the data acquisition system52. For sample separation, the sample can be injected by hydrodynamicinjection or electrokinetic injection methods into capillary 31, and ahigh voltage applied between the ground and power electrodes. Sampledetection is achieved by sensing the potential difference between thereference electrode 42 and the sensing electrode 44 over time. Thesetechniques are described by S. F. Y. Li in Capillary electrophoresis:Principles, Practice and Applications, Elsevier Science Publications,1992.

The embodiment shown in the above figures can be used for conductivitydetection in many methods of electrophoresis. For simplicity, capillaryzone electrophoresis (CZE) is chosen for explaining the principle of thepresent invention. FIGS. 3A and B show a theoretical electric fieldacross the corresponding section of capillary tube 32. FIG. 3A showsbuffer 33 alone. FIG. 3B shows buffer 33 with samples X and Y beingseparated by CZE. When a high voltage is applied, a straight baseline ofelectric field across the whole capillary 32 as shown in FIG. 3A istheoretically obtained because the running buffer is homogeneous duringCZE. However, some difference in the electric field will exist if asample is injected into the capillary. If the sample component'smobility, for example X, is larger than that of the running buffer, theelectric field in the plug of the sample component will be lower thanthat of the running buffer as shown in FIG. 3B. Conversely, if thesample component's mobility, for example Y, is smaller than that of therunning buffer, the electric field in the plug of the sample componentwill be larger than that of the running buffer (FIG. 3B). When thesample components are passing by the region between the fracture 58 andthe outlet of the capillary, the potential between electrodes 44 and 42will change and the analytes A or B can be detected.

A similar design can be used for microchip CE as shown in FIG. 4 andFIG. 5. In this embodiment, only one capillary channel is shown for easeof illustration. It is understood that a CE chip may have numerouschannels with various designs. The microchip CE in this example is madeof two glass plates 60 and 64. On bottom glass plate 60 is fabricatedseparation channel 78, injection channel 80 connected to samplereservoir 82 and 62, and sensor channel 68 connected to sensor reservoir66 and 70. Sample loading electrode 86, waste electrode 90, powerelectrode 88, sensing electrode 72 and ground/reference electrode 74 arefabricated to connect to sample reservoir 82, waste reservoir 62, inletreservoir 84, sensor reservoir 70, and outlet reservoir 76 respectively.On the top glass plate, access holes (not shown) are drilled to accessthe corresponding reservoirs and channels on the glass plate 60. The twoglass plates are bonded together during fabrication. The thickness L1 ofconductive wall 71 between the detection channel 68 and the separationchannel 78 is less than 40 μm, preferably less than 30 μm for boratesilicate glass. Samples are loaded using the loading and wasteelectrodes according to standard methods. It has been shown that a thinlayer of glass is ion conductive. Based on the same principle describedabove for CE, sample components in microchip CE can be detected bymeasuring the potential between the electrodes 72 and 74 duringelectrophoresis. The distance L2 from the detection channel to theoutlet of the separation channel 78 is near or less than the length ofthe sample plug. For a channel made of glass, this thickness ispreferably several tens of micrometers. For microchips made from othertypes of glass or from other material, the thickness of the conductivewall may be determined by one of ordinary skill in the art without undueexperimentation.

Experiments have been done in the laboratory to test the feasibility ofthe present invention. To separate and detect K⁺ and Na⁺, 50 mMtriethanolamine (pH 6.5, adjusted by adding HCl) was used as runningbuffer for CE. Platinum electrodes were used for applying high voltages.The sensing electrodes were Ag/AgCl wire (diameter, 1 mm) electrodes.Gigaohm (GΩ) resistors were chosen for the resistors 48 and 50. Dataacquisition was obtained through a microprocessor. FIGS. 7 and 8 showtypical electropherogram obtained. We can see that K⁺ and Na⁺ ions canbe well separated and detected using the present invention.

From the above explanation, we can expect that noise will exist if highvoltage is used for electrophoresis, and the voltage is not stableduring electrophoresis, as can be seen in the baselines in FIGS. 7 and8. To improve signal /noise ratio (S/N), the ratio of the potentialmeasured to the current generated during CE can be measured using anoise reducing circuit 94. One embodiment is shown in FIG. 6. Thevoltage S1 collected from the sensing electrodes and the voltage S2 dueto the current I are amplified by A2 and A1. Then the signal S isobtained by dividing the output S1′ from A2 by the output S2′ from A1through a divider A3. From FIG. 6 it can be shown that:I=V/R ₀  (1)S ₁ =I×R _(S)×(R ₂/(R ₁ +R ₂₎₎  (2)S ₂ =I×R ₃  (3)S ₁ ′=S ₁×(1+R ₄ /R ₅)  (4)S ₂ ′=S ₂×(1+R ₇ /R ₆)  (5)S=k ₁(S ₁ ′/S ₂′)  (6)From Eq. 1-6S=k ₁(S ₁ ′/S ₂′)=kR _(S)  (7)Where k=k₁×{(1+R₄/R₅)×R₂}/[R₃×(1+R₇/R₆)×(R1+R₂)]=constant

From the above results, one can see the signal S is proportional toR_(S) only and not affected by voltage, current or the resistance of thecircuit. In other words, this improved circuit can remove the effects ofripple of the high voltage power supply. Therefore, the baseline noisecan be reduced and the ratio of signal to noise will improve.

Those skilled in the art will know that many variations of design can berealized based on the same principle as described above. Although aseparate noise reduction circuit 94 is shown in FIG. 6, it should beunderstood by one skilled in the art that other equivalent interfacesare possible. For instance, the noise reducing function of circuit 94can be incorporated into a sophisticated data acquisition system 52 aspart of its internal submodules.

As mentioned above, the distance between the two points where potentialdifference is measured (e.g. L and L2 in FIG. 2B and 5 respectively) ispreferably smaller than the length of the sample plug injected in orderto obtain maximum resolution. For capillary electrophoresis, the lengthof the sample plug Injected is typically around 1 mm. Therefore, L andL2 are preferably less than 1 mm in order to achieve high resolution andsensitivity. Thus good electrical insulation would have to be providedbetween the two measuring points. Alternatively, a channel with asmaller diameter than that of the separation channel may be providedbetween the two measuring points such that the distance therebetween maybe lengthened without compromising resolution and sensitivity. Oneexample is shown in FIG. 9. In this example, the inlet 101 and outletends 107 of a capillary tube 102 is shown. The tube 102 is separatedinto two parts. Section S1, used for separation, has a larger diameterD1, while section S2, proximate the outlet end in this example, has asmaller inner diameter of D2. A sample 109 of length L3 is shown tomigrate from the inlet to the outlet end. As the sample moves towardssection S2, the length of the sample would be lengthened due to thesmaller diameter of the channel. If the two measuring points forpotential gradient detection (which are fracture 103 and the outlet endin this example) is provided at section S2, it is clear that thedistance between these two measuring points may also be proportionatelylengthened.

Capillaries with varying diameters can be made by normal commercialmachines for making capillaries or pulling one end of a capillary tubewith uniform diameter to produce one end with a small diameter afterheating the tube. Commercially available machines include Laser-basedmicropipette pullers, for example the P2000 from Sutter Instrument Co.Channels on microchips having varying sizes can be easily producedthrough different mask design and performing the appropriatephotolithography known in the art. The electrically conductive mediumcontained within the various sensing, outlet and inlet reservoirs may bethe same or different, depending on the applications.

Although fused silica and glass substrates are commonly used asseparation channels in CE and microchip CE, other substrates, such aspoly(dimethylsiloxane) (PDMS) and PMMA, can be used also.

The present invention can be applied to existing electrophoreticchannels by providing conductive elements on them, for example, bybonding some filters on them. The bonding method could be, for example,thermal bonding for many plastics, oxygen plasma bonding for PDMS. For afused silica capillary, well-known techniques such as fracturing, makinga frit (U.S. Reissued Pat. 035102) and applying polymers afterfracturing (U.S. Pat. No. 5,169,510) may all be applied. For glasschannels, a thin wall of 1-40 μm, preferably 1-20 μm, may be used. Themost effective thickness is dependent on the quality of the glass, andmay be determined by one of ordinary skill in the art by routineexperimentation.

The detection channel on microchip CE could be on the top or the bottomof the separation channel rather than lying adjacent to the separationchannel. The electrodes for sensing can be other electrodes, such ascalomel electrode, platinum and gold. The reference electrode In theoutlet reservoir can be combined with the ground electrode. Formicrochip CE, both sensing electrodes and the electrophoresis electrodescan be microfabricated on the chips or just inserted directly in thebuffer reservoirs. It is also possible to create two or more conductiveelements on the capillary or the separation channel in order to detectsample components at different places. For example, by having twofactures along two different points of a capillary tube. The referenceelectrode may also be positioned away from the outlet end by creating anadditional conductive element and the corresponding reservoir forconnection to the reference electrode.

1. An electrophoretic apparatus comprising: a power supply with a groundelectrode and a power electrode; a separation channel with an inlet endand an outlet end and containing separation medium, said inlet endelectrically coupled to the power electrode of said power supply, saidoutlet end electrically coupled to the ground electrode of said powersupply; a data acquisition system with a reference electrode and anelectrical potential sensing electrode, said reference electrodeelectrically coupled to said ground electrode; and a conductive element,provided on said separation channel between said inlet end and saidoutlet end, said conductive element permitting electrical signals topass through without detectable bulk flow of separation medium andsample, said sensing electrode electrically coupled to said conductiveelement whereby the electrical potential within the separation channelbetween the conductive element and the outlet end may be detected bysaid data acquisition system without causing disturbance to the flow ofsamples in said separation channel.
 2. An electrophoretic apparatusaccording to claim 1 further comprising an inlet reservoir connected tosaid inlet end, said inlet reservoir for retaining electricallyconductive medium to which said power electrode is coupled.
 3. Anelectrophoretic apparatus according to claim 1 further comprising anoutlet reservoir connected to said outlet end, said outlet reservoir forretaining electrically conductive medium to which said ground electrodeis coupled.
 4. An electrophoretic apparatus according to claim 1 whereinsaid conductive element is connected to a sensing reservoir such thatsaid conductive element is electrically connected to said sensingelectrode via electrically conductive medium retained within saidsensing reservoir.
 5. An electrophoretic apparatus according to claim 1further comprising an inlet reservoir connected to said inlet end, saidinlet reservoir for retaining said separation medium to which said powerelectrode is coupled; an outlet reservoir connected to said outlet end,said outlet reservoir for retaining said separation medium to which saidground electrode is coupled; and a sensing reservoir connected to saidconductive element, said sensing reservoir for retaining an electricallyconductive medium to which said sensing electrode is coupled.
 6. Anelectrophoretic apparatus according to claim 1 wherein said separationchannel is a capillary tube, and said conductive element is a fracturein said capillary tube.
 7. An electrophoretic apparatus according toclaim 1 wherein said conductive element is 0.1 to 5 mm from the outletend.
 8. An electrophoretic apparatus according to claim 1 wherein saidseparation channel is a capillary tube, said conductive element is afracture in said capillary tube 0.1 to 5 mm from the outlet end.
 9. Anelectrophoretic apparatus according to claim 1 wherein a secondconductive element connected to a sensor reservoir with electricallyconducting medium is provided between said conductive element and saidinlet end, and said reference electrode is electrically connected tosaid second sensor reservoir, such that electrical potential within theseparation channel between said first and second conductive element maybe detected by said data acquisition system without causing disturbanceto the flow of separation medium and samples in said separation channel.10. An electrophoretic apparatus according to claim 1 further comprisinga first resistor coupled between said reference electrode and said dataacquisition system, and a second resistor coupled between said sensingelectrode and said data acquisition system for sampling the potentialdifference between the reference and sensing electrodes.
 11. Anelectrophoretic apparatus according to claim 10 further comprising anoise reducing means interposed between said sensing electrode and saiddata acquisition system for removing the ripple effects of said powersupply, said noise reducing means comprising: at least a first amplifierconnected to the sensing electrode for amplifying the potentialdifference signal between the sensing and the reference electrodes; atleast a second amplifying connected to the ground electrode foramplifying current signals therefrom; and at least one signal dividerhaving its input coupled to said first amplifier and said secondamplifier; the output of said signal divider coupled to said dataacquisition system for removing noise from said potential difference andcurrent signals.
 12. An electrophoretic apparatus comprising: acapillary electrophoresis array chip containing a plurality oflongitudinally aligned capillary separation channels containingseparation medium, each said channel having an inlet end and an outletend, a conductive element provided on each of said separation channelsproximate the corresponding outlet end, said conductive element allowingelectrical signals to pass through without detectable bulk flow of theseparation medium and sample; at least one sensor reservoir provided foreach said conductive element, said sensor reservoir containingelectrically conductive medium and being connected to said conductiveelement of said separation channel; a power supply with a groundelectrode coupled to each of said outlet ends; and a power electrodecoupled to each of said inlet ends for electrophoresis of samplesprovided within said separation channels; and a data acquisition systemwith at least one reference electrode coupled to each of said outletends; and a plurality of electrical potential sensing electrodes eachcoupled to one sensor reservoir whereby electrical potential within saidseparation channels between said conductive element and said outlet endis measured by said data acquisition system.
 13. An electrophoreticapparatus according to claim 12 further comprising a sample channelconnected to each of said separation channel for receiving and loading asample, and a sample electrode coupled to said sample and said powersupply for sample injection.
 14. An electrophoretic apparatus accordingto claim 12 further comprising an inlet reservoir connected to each ofsaid inlet end, said inlet reservoir for retaining separation medium towhich said power electrode is coupled.
 15. An electrophoretic apparatusaccording to claim 14 wherein said inlet reservoirs are interconnected.16. An electrophoretic apparatus according to claim 12 furthercomprising an outlet reservoir connected to each of said outlet end,said outlet reservoir for retaining separation medium to which saidground electrode is coupled.
 17. An electrophoretic apparatus accordingto claim 16 wherein said outlet reservoirs are interconnected.
 18. Anelectrophoretic apparatus according to claim 12 wherein said conductiveelement is connected to a sensing reservoir such that said conductiveelement is electrically connected to said sensing electrode viaelectrically conductive medium retained within said sensing reservoir.19. An electrophoretic apparatus according to claim 12 furthercomprising an inlet reservoir connected to each of said inlet end, saidinlet reservoir for retaining separation medium to which said powerelectrode is coupled; an outlet reservoir connected to each of saidoutlet end, said outlet reservoir for retaining said separation mediumto which said ground electrode is coupled; and a sensing reservoirconnected to said conductive element, said sensing reservoir forretaining an electrically conductive medium to which said sensingelectrode is coupled.
 20. An electrophoretic apparatus according toclaim 19 wherein said outlet reservoirs are interconnected.
 21. Anelectrophoretic apparatus according to claim 19 wherein said inletreservoirs are interconnected.
 22. An electrophoretic apparatusaccording to claim 12 wherein said electrophoretic chip is made ofglass, and said conductive element is a thin wall separating saidchannel and said sensor reservoir, said thin wall being less than 40 μmthick.
 23. An electrophoretic apparatus according to claim 12 whereinsaid conductive element is less than 5 mm from the outlet end.
 24. Anelectrophoretic apparatus according to claim 12 wherein said conductiveelement is a thin wall separating said channel and said sensorreservoir, and the length of said conductive element along thelongitudinal axis of said channel is less than 10 mm.
 25. Anelectrophoretic apparatus according to claim 12 wherein a secondconductive element connected to a second sensor reservoir withelectrically conducting medium is provided on said channel between saidconductive element and said inlet end, and said reference electrode iselectrically connected to said second sensor reservoir, such thatelectrical potential within the separation channel between said firstand second conductive elements may be detected by said data acquisitionsystem without causing disturbance to the flow of separation medium andsamples in said separation channel.
 26. An electrophoretic apparatusaccording to claim 12 further comprising a sample channel connected tosaid separation channel for receiving and loading a sample; a sampleelectrode in electrical contact with said sample in said sample channeland coupled to said power supply for sample injection; a waste channelconnected to said sample reservoir for receiving and retaining excesssamples; and a waste electrode in electrical contact with said sample insaid waste channel and coupled to said power supply for controlling thesample injection process.
 27. A method for detecting samples in anelectrophoretic system, said system containing an electrophoreticchannel with an inlet end and an outlet end, said channel containingelectrophoretic medium and samples, said apparatus further containing anelectrically conductive element on the wall of said electrophoreticchannel proximate said outlet end, said conductive element permittingelectrical signal to pass through without detectable bulk flow ofelectrophoretic medium and sample, said method comprising: separatingthe samples in said electrophoretic channel containing electrophoreticmedium by producing an electrical field between the inlet end and theoutlet end; sensing the electrical potential within the channel betweenthe conductive element and the outlet end.